This invention relates to an apparatus for solder joining metal tapes in a continuous tape laminating apparatus.
Soldering has been defined as metal coalescence below about 427.degree. C. As a result, soldering facilitates joining parts while minimizing damage from heating. Solder alloys are comprised of tin and lead, and the tin component reacts with metals to be joined to form a metallurgical bond. In many soldering systems, an intermetallic compound is developed at the interface between the solder and the base metal, producing an essentially complete metallurgical joint. The cleanliness and chemical composition of the surfaces to be joined are critical to the process. For example, flux can be used to insure that the base metal is sufficiently cleaned to provide adequate spread and flow of the soldering alloy to promote joint formation. However, when a flux is not used surface cleanliness, e.g., freedom from surface oxides, is critical to joint formation.
One application that has been disclosed for solder joined metal tapes is for laminated superconducting tapes, for example disclosed in U.S. Pat. No. 3,537,827. Briefly described, the laminated superconducting tape is comprised of a brittle triniobium tin superconducting layer bonded between outer laminae of non-superconductive metals having a coefficient of thermal expansion greater than that of the triniobium tin inner layer. The outer laminae are bonded integrally to each side of the triniobium tin inner layer by soldering. As a result, the triniobium tin inner layer is in a state of mechanical compression which results from the fact that the outer laminae are in a state of mechanical tension.
Uniform solder joining of the outer laminae to the relatively brittle inner layer provides benefits, such as, improved thermal conductivity for cooling of the superconducting core of the tape, improved formability of the tape, and greater resistance to handling damage. For some applications the laminated tape must have a uniform cross section, for example to enable uniform winding of the tape to form coils. Therefore, it is desirable to solder the metal tapes to form a uniform solder joint, and a laminated tape having a uniform cross-section with a smooth surface finish in a continuous operation. However, imperfections in the tapes that are to be laminated such as camber or wavy edge cause misalignment of the tapes in the width dimension, oxidized tape surface causes dewetting and poor solder bonding, and particles in the solder such as dross (tin oxide) or intermetallic compounds (CuSn) can cause non-uniform cross-sections and excessively rough surface finish defects in the laminated tape.
Excessive time at the soldering temperature, about 250.degree. C., can produce softening of the outer laminate. Since yielding of the outer laminate will cause fracture of the relatively brittle superconductive inner laminate, it is desirable to maintain the yield strength of the outer laminate as high as possible. Therefore it is desirable to minimize the time the tapes are at the soldering temperature.
In copending application Ser. No. 07/767,448, filed Jan. 28, 1992, incorporated herein by reference, an apparatus for soldering metal tapes to form laminated tapes is disclosed. The apparatus is comprised of an alignment box defining channels therein for aligning the tapes in their width dimension. The apparatus has been used to laminate copper tapes to a triniobium tin inner laminate. To minimize oxide formation on the surface of the copper tapes, the copper tapes are precoated with electroplated solder. However, the electroplated solder coating can have a carbonaceous organic residue, and an oxide film on the surface thereof.
The organic residue and oxide film in the electroplated solder coating are washed from the surface of the copper tape with molten solder during the lamination process. A significant accumulation of particles from the organic residue and oxide occurs in the channels of the alignment box during the lamination of long lengths of tapes. Eventually, the channels can become packed with the particulate buildup, causing the tapes to jam in the channels. This not only stops the lamination process, but can lead to breakage of the tapes that are being laminated. In addition, it is possible for agglomerated particles to breakaway from the accumulation in the channels and become trapped within the laminated tape, forming unacceptable defects in the laminated tape.
One aspect of this invention is to provide an apparatus for solder joining metal tapes having means for washing particles from the apparatus to minimize particle buildup therein.